The Collaborative International Dictionary of English v.0.48:

Polyconic \Pol`y*con"ic\, a. [Poly- + conic.]
   Pertaining to, or based upon, many cones.
   [1913 Webster]

   Polyconic projection (Map Making), a projection of the
      earth's surface, or any portion thereof, by which each
      narrow zone is projected upon a conical surface that
      touches the sphere along this zone, the conical surface
      being then unrolled. This projection differs from conic
      projection in that latter assumes but one cone for the
      whole map. Polyconic projection is that in use in the
      United States coast and geodetic survey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polyconic projection
    n 1: a conic projection of a map having distances between
         meridians equal to those distances on a globe